If you move your fingers you can create false heart rate measurements (finger movement artifacts). Also if you are cold or have poor circulation the finger sensors may have trouble reading your heart rate. In either case faulty readings appear as the heart rate jumping directly to about twice or half its value (from 50 directly to 100 beats per minute for example). Try keeping your fingers still, putting on a sweater, or turning on the heat in your home. You can also warm your hands by relaxing, as relaxation tends to lead to warmer hands through increased blood flow to your hands and feet.